Current activities in the U.S. and Canadian coal bed methane (CBM) plays are near a standstill due to the depressed natural gas market, with Henry Hub prices averaging less than $4.50/Mcf. However, shale play activities (e.g., Bakken (MT and ND), Marcellus (PA and NY), Barnett, and Eagle Ford (TX)) are undergoing rapid development with no signs of slowing down.
The U.S. Geological Survey estimates mean undiscovered volumes of 3.65 billion barrels of oil, 1.85 trillion cubic feet of associated and dissolved natural gas, and 148 million barrels of natural gas liquids in the Bakken Shale Formation of the Williston Basin Province, Montana and North Dakota (http://geology.com/usgs/bakken-formation-oil.shtml). Of this resource, the Bakken Shale Play underlies 11 Montana counties, including Daniels, Dawson, Fallon, Garfield, McCone, Prairie, Richland, Roosevelt, Sheridan, Valley, and Wibaux.
However, inability to economically manage or dispose of the high total dissolved solids (TDS) frac-return waters produced during shale play development is a costly impediment to resource extraction due to both transportation and disposal costs. High TDS waters exhibit greater than 5000 ppm. Additionally, ever increasing regulation of these produced and frac-water discharges threaten economic development of fossil resources. A recent article in the New York Times, entitled “Regulation Lax as Gas Wells' Tainted Water Hits Rivers—“We're burning the furniture to heat the house” (http://www.nytimes.com/2011/02/27/us/27gas.html?_r=2&hp) is one of many examples regarding the difficulties presented in disposing of these high TDS waters. The movie “Gasland” (http://www.gaslandthemovie.com) provides graphic examples of the regulatory and material issues related to both production and disposal of shale play produced and frac-return waters.
With the advent of horizontal drilling and fracturing of shale plays, large quantities of extremely high TDS frac-return waters are now produced in regions where disposal and recycle options are extremely limited.
In any event, before ultimate disposition, (e.g., deep hole injection, recycle, reuse, conversion to beneficial use, or discharge to surface waters) produced water and frac-return water usually must be conditioned by removal of some or nearly all TDS. Most conventional treatment processes (e.g., evaporation, distillation, reverse osmosis, electrodialysis, ion exchange, etc.) are merely water separation processes that generate a larger volume of low-TDS product water and a smaller volume of high-TDS concentrate or brine—the high-TDS concentrate or brine often requires costly disposal.
At present, there are three major methods in use for ultimate disposal of high-TDS aqueous fluids; injection into geologic formations, natural evaporation, and forced evaporation. Successful injection into an adjacent formation is only possible if there exists an aquiclude or substantial aquitard between the pumped formation and the injected formation. Absent such a confining geologic formation, the injected water will simply flow back to the pumped wells and the net effect is to pump water in a circle.
High TDS fluids can also be transported for commercial disposal or other disposal via a Class II injection well. However, such disposal options are typically not universally applicable and economically viable. For instance, economic disposal via Class II injection wells often entails: (1) existence of an appropriate receiving formation; (2) construction and permitting of the well and surface facilities for surge storage water analysis and chemical and physical water adjustment and high-pressure injection; (3) propinquity of the source of fluid to the injection well site, and existence of transportation infrastructure and services as needed to ensure reasonable transportation costs; (4) compatibility of the injected fluid with the receiving formation; and (5) continued availability and capacity of disposal services.
Evaporation of high TDS fluids to dryness may be effected by a number of means. If climate, terrain, capacity, and regulations allow, high-TDS fluids can be put in a pit or pond (usually lined) for natural or enhanced (e.g., spray, aeration, etc.) evaporation. In the rare cases where natural evaporation is feasible, it may be a good, cost-effective means of drying salt solutions. It is only feasible, however, at sites where the annual pan evaporation rate substantially exceeds the annual precipitation rate. That means only arid regions or actual deserts are normally suitable for use of natural evaporation. Even then the technology is not free. Impoundments must be lined, and often they must be fenced and netted in order to prevent wildlife intrusion. Finally, since evaporation only occurs at the surface of the impoundment, evaporation ponds usually exhibit a large surface area for the amount of water evaporated. Hence, natural evaporation is also not an effective and generally applicable option for high-TDS fluid disposal.
As an alternative to these methods, forced evaporation, or evaporation via man-made heat sources, has been attempted by many vendors and service providers. Evaporation of water is energy intensive, and most thermal processes for treating high-TDS fluids employ some type of vapor recompression, multiple effect, or countercurrent flash technology to reduce energy consumption. Unfortunately, these evaporation/condensation schemes employ relatively small temperature differences across the evaporator/condenser heat exchanger surfaces. Consequently, extended heat transfer surfaces, which are expensive to fabricate, are required for reasonable throughput.
Extended heat transfer surfaces include designs that maximize the ratio of surface area to volume, and can include structures such as closely spaced tubes, spiral or corrugates plates, fins, pins, baffles, and expansion joints, to name a few. In addition, to prevent corrosion and stress corrosion cracking, high-alloys and exotic materials are typically employed (e.g., Hastelloy, Inconel, C-276, titanium, etc.). The combination of the extended heat transfer surface and the high alloy and exotic materials greatly increases the size and capital cost of facilities carrying out forced evaporation of high-TDS fluids.
There is therefore a need in the art for affordable, efficient, and mobile zero liquid discharge (ZLD) treatment technology for high-TDS waters generated during oil and gas production. Preferably such water is suitable for unrestricted discharge to surface waters and for other beneficial uses, such as irrigation, aquaculture, and land application.